The tendency of metals to corrode has long been a recognized concern. By corrosion is meant the degradation of a metal by the environment by either chemical or electrochemical processes. A large number of crystalline alloys have been developed with various degrees of corrosion resistance in response to various environmental conditions under which the alloys must perform. As examples, stainless steel contains nickel, chromium and/or molybdenum to enhance its corrosion resistance. Glass and metals such as platinum, palladium, and tantalum are also known to resist corrosion in specific environments. The shortcomings of such materials lie in that they are not entirely resistant to corrosion and that they have restricted uses. Tantalum and glass resist corrosion in acidic environments but are rapidly corroded by hydrogen fluoride and strong base solutions.
The corrosion resistance of an alloy is found generally to depend on the protective nature of the surface film, generally a passive oxide film. In effect, a film of a corrosion product functions as a barrier against further corrosion.
"Corrosion and Electrochemical Behavior of Chromium-Nobel Metal Alloys", J. Electrochem. Soc., 1961, Vol. 108, No. 9, pp 836-841, by Greene et al., presents a discussion of alloying chromium with small amounts of platinum, palladium, indium, rhodium, ruthenium, or osmium to produce crystalline alloys with improved corrosion resistance. These crystalline alloys were tested in boiling H.sub.2 SO.sub.4, HCl and HNO.sub.3, and demonstrated improved corrosion resistance in dilute nonoxidizing acids.
In recent years, amorphous metal alloys have become of interest due to their unique characteristics. While most amorphous metal alloys have favorable mechanical properties, they tend to have poor corrosion resistance. An effort has been made to identify amorphous metal alloys that couple favorable mechanical properties with corrosion resistance. Amorphous ferrous alloys have been developed as improved steel compositions. Binary iron-metalloid amorphous alloys were found to have improved corrosion resistance with the addition of elements such as chromium or molybdenum, M. Naka et al, Journal of Non-Crystalline Solids, Vol. 31, page 355, 1979. Naka et al. noted that metalloids such as phosphorous, carbon, boron and silicon, added in large percentages to produce the amorphous state, also influenced its corrosion resistance.
T. Masumoto and K. Hashimoto, reporting in the Annual Review of Material Science, Vol. 8, page 215, 1978, found that iron, nickel and cobalt-based amorphous alloys containing a combination of chromium, molybdenum, phosphorus and carbon were found to be extremely corrosion resistant in a variety of environments. This has been attributed to the rapid formation of a highly protective and uniform passive film over the homogeneous, single-phase amorphous alloy which is devoid of grain boundaries and most other crystalline defects.
Many amorphous metal alloys prepared by rapid solidification from the liquid phase have been shown to have significantly better corrosion resistance than their conventionally prepared crystalline counterparts, as reported by R. B. Diegel and J. Slater in Corrosion, Vol. 32, page 155, 1976. Researchers attribute this phenomena to three factors: Structure, such as grain boundaries and dislocations; chemical composition; and homogeneity, which includes composition fluctuation and precipitates.
Ruf and Tsuei reported amorphous Cr-B alloys having extremely high corrosion resistance, "Extremely High Corrosion Resistance in Amorphous Cr--B Alloys", Journal of Applied Physics, Vol. 54 No. 10, p. 5705, 1983. Amorphous films of Cr--B alloys containing from about 20 to 60 atomic percent boron were formed by rf sputtering. At room temperature, Ruf and Tsuei reported that in 12N HCl high corrosion resistance was observed only when boron as present in the amorphous alloy at between 20 and 40 atomic percent. Bulk polycrystalline Cr was reported to dissolve at about 700 millimeters/day in 12N HCl at room temperature.
A thorough discussion of the corrosion properties of amorphous alloys can be found in Glassy Metals: Magnetic, Chemical, and Structural Properties, Chapter 8, CRC Press, Inc., 1983. In spite of advances made to understand the corrosion resistance of amorphous metal alloys, few alloys have been identified that exhibit little or no corrosion under extremely harsh acidic and/or alkaline environments. Those few alloys which do exhibit such properties utilize expensive materials in the alloy composition and so are prohibitive for many applications where their properties are desired.
Amorphous metal alloys that have been studied for corrosion resistance and have been evaluated under relatively mild conditions, 1N-12N HCl, and at room temperature. However, under more severe conditions, such as 6.5N HCl at elevated temperatures, those amorphous metal alloys cited as having good corrosion resistance may not be suitable for use.
What is lacking in the field of amorphous metal alloys are economical alloy compositions that exhibit a high degree of corrosion resistance under severely corrosive conditions.
It is, therefore, one object of the present invention to provide amorphous metal alloy compositions having excellent corrosion resistance in oxidizing and nonoxidizing acid environments.
It is another object of the invention to provide such amorphous metal alloy compositions in a cost-effective manner.
These and other objects of the present invention will become apparent to one skilled in the art of the following description of the invention and in the appended claims.